WASHINGTON (CNN) -- The discovery of an unexploded bomb in a parked London car poses no specific threat to the United States, the FBI said Friday, although it's urging police and the public to remain vigilant.

NYPD officers wait to go on patrol in Times Square, New York, on Friday.

The Department of Homeland Security said it has no plans to change the U.S. terrorism threat level in reaction to the incident, which involved a car police said was packed with nails and gasoline.

In a written statement, Homeland Security Secretary Michael Chertoff said, "I have seen no specific, credible information suggesting that this incident is connected to a threat to the homeland."

Chertoff mentioned the upcoming Independence Day holiday but asked the public, "as always, that they be vigilant and report any suspicious activities to authorities."

He said Homeland Security officials have been in close contact with UK authorities.

The current "elevated" threat level -- color coded as yellow -- is defined as a "significant risk of terrorist attacks." Separate threat levels for New York City and U.S. aviation remain at "orange" or "high."

Scotland Yard described the find as a "potentially viable explosive device" that was discovered in a silver Mercedes-Benz sedan near Piccadilly Circus in central London.

President Bush has been briefed on the situation by national security adviser Stephen Hadley, and the White House also is monitoring, said spokesman Tony Snow.

At the Pentagon, the car bomb was mentioned during a scheduled meeting of the Joint Chiefs of Staff, where officials discussed making sure that necessary authorities would be informed of the incident.

An FBI statement said the agency plans to follow all appropriate leads, although "at this time we have no specific information on any threats related to this incident here in the U.S."

The statement appeals for the public to "remain vigilant and report suspicious activity to law enforcement."

Also, state, local and federal law enforcement officials in the United States are being urged to be alert.

New York City police said that as a precaution, they are assigning officers to subways and implementing additional vehicle checkpoints throughout the city.

"We'll take a little bit of extra precaution," said New York Mayor Michael Bloomberg. "We're going to ramp up a little bit ... nothing dramatic ... some you'll notice, some you won't."

The mayor said the city has a police officer assigned full time to London who will keep authorities posted on the British investigation.

In Washington, a few extra police officers were added to the streets of the nation's capital, said Kenneth McCracken, spokesman for the city's Homeland Security and Emergency Management Agency.

Police in other U.S. cities including Los Angeles, California; Atlanta, Georgia; Miami, Florida; and New Orleans, Louisiana; said they also were monitoring the situation, although they had no plans to increase security.

Metropolitan Police Deputy Assistant Commissioner Peter Clarke said it was too soon to speculate on who was behind the car bomb and what the target might have been.

"In the car, they found significant quantities of [gasoline] together with a number of gas cylinders," Clarke said. While he could not immediately say how much fuel was there, one source told CNN it added up to about 50 gallons -- 100 liters in the front and 100 liters in the back.

"I can tell you it was in several large containers," Clarke said. "There were also a large number of nails in the vehicle."

Clarke said it's "obvious that if the device had detonated there could have been significant injury or loss of life." E-mail to a friend